The present invention relates to a photolytic device for organic chlorine compounds. More particularly, the present invention relates to a photolytic device for organic chlorine compounds, wherein organic chlorine compounds in gaseous form are broken down and removed by ultraviolet irradiation. The organic chlorine compounds may be obtained through vacuum extraction of soil polluted with organic chlorine compounds. Alternatively, the organic chlorine compounds may be obtained through aeration of ground water or waste gas from factories. By this process, organic chlorine compounds are efficiently removed at a high breakdown rate and with a minimal amount of electric power expenditure.
Organic chlorine compounds have been used for many years as dissolving agents, and display excellent dissolving characteristics. For example, tetrachloroethylene and trichloroethylene have a strong dissolving power with respect to fats and oils, and are excellent flame retardants. In recent years, however, it has been shown that many such organic chlorine compounds are carcinogenic, and release of these compounds into the environment has been restricted. However, the problem of existing pollution of the environment by such organic chlorine compounds has still not been solved. These substances have been detected from wells in many areas and countries, and the pollution of ground water and soils with organic chlorine compounds has become a large problem.
Because organic chlorine compounds are readily volatile, they may spread easily in gaseous or vaporous form. Organic chlorine compounds are conventionally extracted as a mixture with air by several different methods. In the case of pollution of soil by organic chlorine compounds, the main conventional method currently employed is a vacuum extraction method. In the case of pollution of ground water by organic chlorine compounds, the principal conventional method involves first pumping the ground water, followed by aeration of the ground water. In the case of a vacuum extraction method, the concentration of organic chlorine compounds in the vacuum chamber can reach several thousand to several tens of thousands ppm. In either case, after vacuum extraction from soil or after pumping up and aerating ground water, organic chlorine compounds are recovered by adsorbing the organic chloride compounds, using an adsorbing agent such as activated carbon or the like, and the complex of adsorbing agent and organic chlorine compound is further processed as solid waste. However, this type of approach does not destroy the pollutant; it simply transfers the site of pollution to a waste site.
Several techniques have been proposed to break down organic chlorine compounds on site, and thereby render them harmless. For example, in Japanese Laid Open Patent Number 62-191025, there is proposed a treatment method for waste gas containing organic halogen compounds, wherein waste gas which contains organic halogen compounds is stirred and simultaneously exposed to ultraviolet radiation. As a result, the organic halogen compounds are converted into an acidic breakdown gas. The acidic breakdown gas is then washed with an alkali solution, creating a harmless breakdown product. In Japanese Laid Open Patent Number 62-191095, there is proposed a waste water treatment device equipped with an aeration processing device which aerates waste water; an ultraviolet light treatment device which treats the gas discharged from the aeration treatment device with ultraviolet light; and an alkaline wash device which treats the gas discharged from the ultraviolet treatment device with an alkali solution. Furthermore, in Japanese Laid Open Patent Number 1-236925, there is proposed a breakdown device for halogenated ring hydrocarbons, wherein halogenated ring hydrocarbons and ozone are first brought to a breakdown space and irradiated with ultraviolet light, thereby breaking down the halogenated ring hydrocarbons. In a second step, harmful substances within the breakdown generated gas are removed. In Japanese Laid Open Patent Number 2-75391, there is proposed a waste water treatment device equipped with a reaction device wherein waste water which contains organic halides is first aerated and irradiated with ultraviolet light. The waste water treatment device also includes an ultraviolet irradiation treatment device which irradiates with ultraviolet light the first waste gas discharged from the reaction device. Finally, an alkaline wash device treats the second waste gas discharged from the ultraviolet irradiation treatment device with an alkali.
Because organic chlorine compounds are relatively easily broken down photochemically, many such breakdown methods and devices which employ ultraviolet radiation have been proposed. However, it is difficult to completely break down organic chlorine compounds via a photoreaction that employs ultraviolet irradiation alone. There are often residual reaction intermediate products which contain chlorine atoms. Because of this, there is proposed in Japanese Laid Open Patent Number 7-116467 a treatment method for volatile organic compounds, wherein volatile organic compounds contained in soil or in water are first extracted by vacuum or aeration. The extracted compounds are then oxidized in the presence of oxygen by irradiation with ultraviolet rays. The oxidized compounds are then further broken down by bringing the gas which contains oxidized products into contact with water and exposing the oxidized products to aerobic organisms. However, this type of treatment method, which combines breakdown by a photolytic reaction through ultraviolet irradiation with breakdown by microorganisms, generates reaction intermediate products which have deleterious effects on microorganisms. As a result, it is difficult to conduct stable processing of organic halogen compounds over an extended period of time by a process that includes microorganisms.
Within Japan, sites which are polluted with organic chlorine compounds and which are in need of cleanup are often of a small lot size. It is difficult to employ a large cleaning device. As a result, a compact, high-performance cleaning device is desirable. Furthermore, when the target compounds are in gaseous or vaporous form and have been vacuum-extracted from soil, the concentration of organic chlorine compounds present in the gas or vapor will, over time, gradually decrease from the start of cleaning. However, this concentration fluctuates from day to day. A cleaning device which can stably process organic chlorine compounds, even with a fluctuating load, is desirable. In addition, the photolytic reaction of organic chlorine compounds has not necessarily been adequately analyzed. There still remains the question of how to optimize the photolytic reaction. A general method for optimizing a photoreactive container where ultraviolet irradiation can take place has not yet been developed. Therefore, to accommodate fluctuating concentrations of organic chlorine compounds, a large reaction container is often employed. However, a large reaction container is not optimal, for at least the reason that a large amount of electrical power is required to break down the organic chlorine compounds. Because a large amount of electrical power is required, the ultraviolet breakdown method, which theoretically should be highly efficient, is not yet economically feasible.